There are three types of electrolytic cells primarily used for the commercial production of halogen gas and aqueous alkali metal hydroxide solutions from alkali metal halide brines, a process referred to by industry as a chlor-alkali process. Two of these cells are the diaphragm cell and the membrane cell. The general operation of each cell is known to those skilled in the art and is discussed in Volume 1 of the Third Edition of the Kirk-Othmer Encyclopedia of Chemical Technology at page 799 et. seq; the relevant teachings of which are incorporated herein by reference.
In the diaphragm cell, an alkali metal halide brine solution is continually fed into an anolyte compartment containing an anolyte solution where halide ions are oxidized at the anode to produce halogen gas. The anolyte solution, including alkali metal cations contained therein, migrates to a catholyte compartment containing a catholyte solution through a hydraulically-permeable microporous diaphragm disposed between the anolyte compartment and the catholyte compartment. Hydrogen gas and an aqueous alkali metal hydroxide solution are produced at the cathode. Due to the hydraulically permeable nature of the diaphragm, the anolyte solution mixes with the alkali metal hydroxide solution formed in the catholyte compartment.
The membrane cell functions similarly to the diaphragm cell, except that the diaphragm is replaced by a hydraulically-impermeable, cationically-permselective membrane which selectively permits passage of alkali metal ions to the catholyte compartment. The membrane essentially prevents hydraulic permeation of the anolyte solution to the catholyte compartment, except for the alkali metal cations. Therefore, a membrane cell produces alkali metal hydroxide solutions relatively uncontaminated with the alkali metal halide brine.
Membrane cells are typically assembled in "stacks" comprising a plurality of bipolar plate electrodes, the electrodes being assembled in a filter press arrangement wherein each electrode is positioned in a spaced-apart but face-to-face planar relationship with respect to an adjacent electrode. A membrane is positioned between each adjacent bipolar electrode, thereby forming a series of alternating catholyte and anolyte compartments. A stack may also comprise a plurality of membrane cells having monopolar electrodes where the cells are electrically connected in series with respect to each other. Membrane cell stacks generally have common electrolyte and product piping. Membrane cell stacks are known in the chlor-alkali industry and, for example, are described in Volume 6A of Ullman's Encyclopedia of Industrial Chemistry (5th Ed. 1986) at pages 399 et seq; the relevant teachings of which are incorporated herein by reference.
During normal operation of a chlor-alkali membrane cell stack, electric current flows from the anode to the cathode in a cell which places the cathode at a negative potential, typically around -1.0 volts versus a mercury/mercuric oxide reference electrode. As used hereinafter, the term "normal positive current flow" refers to the current flow which is impressed by a power source. i.e. a rectifier, external to the cell in order to conduct electrolysis. When normal positive current flow to the cell is interrupted, a membrane cell essentially functions as a battery and may discharge by a flow of electric current in a direction opposite that of the normal positive current flow. As used hereinafter, the term "reverse current flow" refers to the electrical current which flows due to cell discharge after interruption of the normal positive current flow. During reverse current flow, the cathode potential shifts in a positive direction and may rise to a level that leads to cathode corrosion.
It should be understood that the terms "cathode" and "anode" as used herein refer to electrodes having those respective functions during normal cell operation. Normally, reduction is conducted at the cathode, while oxidation is carried out at the anode. However, during reverse current flow, electrode function is reversed from that which prevails during normal operation. For example, although an electrode is a cathode during normal operation, it is an anode in an electrochemical sense during reverse current flow. To avoid potential confusion hereinafter, the terms "cathode" and "anode" refer to electrodes having these respective functions during normal operation, regardless of which direction the electric current is flowing at a given point in time.
In a chlor-alkali cell used to electrolyze, for example, a sodium chloride brine, it is believed that reverse current flow is promoted by electrochemical reactions. Oxidation of adsorbed hydrogen gas on the cathode occurs according to the following reaction: EQU H.sub.2 +2OH.sup.- .fwdarw.2H.sub.2 O+2e.sup.-
while reduction of dissolved chlorine gas, oxygen gas, hypochlorous ion and chlorate ion occurs at the anode according to the following reactions: EQU Cl.sub.2 +2e.sup.- .fwdarw.2Cl.sup.- EQU 4H.sup.+ +O.sub.2 +4e.sup.- .fwdarw.2H.sub.2 O EQU OCl.sup.- +H.sub.2 O+2e.sup.- .fwdarw.Cl.sup.- +2OH.sup.- EQU ClO.sub.3.sup.- +3H.sub.2 O+6e.sup.- .fwdarw.Cl.sup.- +6OH.sup.-
It is believed that reverse currents promoted by the above chemical reactions are conveyed through electrically conductive cell piping, such as common anolyte and catholyte inlet manifolds (also known in the art as a "header") and related piping associated with a membrane cell stack. See, e.g., H. S. Burney et al., "Predicting Shunt Currents in Stacks of Bipolar Plate Cells with Conducting Manifolds", 135 J. Elec. Chem. Soc. 1609-1612 (July 1988) and R. E. White et al., "Predicting Shunt Currents in Stacks of Bipolar Plate Cells", 133 J. Elec. Chem. Soc. 485-492 (March 1986); the relevant teachings of which are incorporated herein by reference. The reverse current flow is also believed to be conveyed electrolytically by electrolytes contained in such manifolds and related piping.
It is known in the art that platinum group metals, such as ruthenium, rhodium, osmium, iridium, palladium, platinum, as well as the oxides of the platinum group metals, are useful as electrocatalysts in electrochemical reactions. Electrodes may be fabricated from such electrocatalysts, but a more economical practice is to coat a substrate with a layer of suitable electrocatalysts Electrodes incorporating such electrocatalysts reduce power consumption and are widely used in various forms by industry. Examples of such electrodes appear in U.S. Pat. No. 4,760,041.
One problem associated with development of reverse current flow in membrane electrolytic cells is galvanic corrosion of electrodes, such as cathodes and electrocatalytic coatings thereon. For example, it is believed that as the above-identified chemical reactions proceed and promote reverse current flow in a chloralkali cell, a point is eventually reached where essentially all hydrogen gas available for oxidation, i.e., hydrogen gas that is either adsorbed on the cathode surface or dissolved in the catholyte solution, is consumed. Due to a higher solubility of chlorine gas in the anolyte in comparison to hydrogen gas in the catholyte, a larger amount of chlorine gas is available for reduction at the anode in comparison with hydrogen gas available for oxidation at the cathode. Accordingly, reduction of chlorine-based chemical agents that include, for example chlorine gas, chlorate ion and hypochlorous ion, at the anode continues after depletion of the hydrogen gas with a corresponding oxidation (corrosion) of electrocatalyst coatings, such as ruthenium dioxide, at the cathode. As used herein, the term "galvanic corrosion" refers to the above-described corrosion problem.
Galvanic corrosion can occur shortly after loss of electrical power to the cell stack or during initial start-up of the stack. When normal positive current flow to a membrane cell stack is interrupted due to loss of electrical power or a maintenance problem during operation, cathodes are observed, in many instances, to rapidly corrode. Within a short period of time, i.e., often less than about an hour for a cell stack having 30 or more cells, hydrogen gas adsorbed on the cathode is consumed, and thereafter, a rapid, positive, increase in cathode potential occurs until the cathode surfaces begin to corrode. Galvanic corrosion may occur during initial start-up of the cell stack, but it is generally not as severe as during interruptions in normal cell operation. Galvanic corrosion is likely in cells located toward the center of a membrane cell stack consisting of about ten or more cells, and is particularly severe where the stack consists of about 30 or more cells.
As used hereinafter, the term "corrosion potential" means the equilibrium potential, i.e., an oxidation half cell potential, for the particular material from which the cathode is fabricated. For example, where ruthenium dioxide is used as an electrocatalytic cathode coating, the oxidation half cell reaction may be represented by: EQU 4OH.sup.- +RuO.sub.2 .fwdarw.RuO.sub.4.sup.= 30 2e.sup.- +2H.sub.2 O
The equilibrium potential for this oxidation half cell reaction is about +0.1 volts versus a mercury/mercuric oxide reference electrode. As the cathode potential nears this equilibrium potential, corrosion is observed to occur.
Loss of the electrocatalyst is undesirable for commercial operation of membrane electrolytic cells. Catalyst loss increases the cell voltage required for normal operation and thereby results in greater power consumption. In severe cases of corrosion, replacement of the cathode may be required which is also economically undesirable due to the labor and material costs associated with the replacement.
It is also believed that reverse current flow may damage the membrane associated with cells in a stack. Reverse current flow may change the chemical characteristics of the catholyte solution and cause precipitation of chemical species in the membrane.
As a result, it is desirable to develop methods of controlling reverse current flow in membrane electrolytic cells while the cells are out of operation due to, for example, loss of electrical power, process maintenance problems or initial cell start-up. An object of the present invention is to control reverse current flow and its attendant problems.